Envelope protein VP28 has been suggested as a candidate vaccine component to evoke a better protection against white spot syndrome virus (WSSV), a major shrimp pathogen.In our previous papers, we reported that Bacillus subtilis spores harboring VP28 (rVP28-bs) can specifically protect shrimp against WSSV by oral delivery.However, the mechanism that supports the production of unique molecules induced by rVP28-bs to trigger specific immune responses against WSSV is originally unknown.It has recently been suggested that Dscam (Down syndrome cell adhesion molecule),a member of the immunoglobulin superfamily (IgSF), plays an essential role in the alternative adaptive immune system of invertebrates.In this study, we compared the diversity of Litopenaeus vannamei Dscam (LvDscam) variable regions and isoform populations by different antigens immunization (rVP28-bs, B.subtilis and Lipopolysaccharide).A total of 13, 15and 11 expressed alternative sequences were identified for N-terminal Ig2, N-terminal Ig3 and the entire Ig7 domain,respectively, which implies that LvDscam can potentially encode at least 2145 isoforms at this experimental condition.More than half of the unique variants (16 out of 22) were found in the Ig2/Ig3 domains.Further analysis of the interaction between VP28 and unique Ig2/Ig3 variants was confirmed by yeast two-hybrid approach.Moreover, we found that the percentage of haemocytes phagocytosing WSSV was significantly higher (p< 0.001) in shrimp injected with control-siRNA (43.8 ± 2.2) than those injected with Dscarn-siRNA (11.3 ± 5.4) in the rVP28-bs immunized groups.With Dscam-siRNA injection, the number of survivors significantly decreased (p< 0.001) in the rVP28-bs immunized group after WSSV challenge compared with those with control-siRNA injection.Our data suggested that LvDscam-mediated pathway may be involved in the specific immune response of shrimp against WSSV induced by rVP28-bs.